1. Field of the Invention
The present invention relates to a solid image pickup element and its driving method intended for decreasing a smear.
2. Description of the Prior Art
Because of the remarkable improvements taking place in solid image pick up elements, and the resulting refinement in area per pixel, operation at higher sensitivities is now required. To satisfy such requirements, a charge sweep device type image pickup element (hereinafter referred to as CSD type image pickup element) disclosed at page 100, February issue of DIGEST OF TECHNICAL PAPERS 1985, International Solid State Circuit Conference (ISSCC) by Kimata et al. has been developed.
FIG. 6 is a block diagram showing a driving device for such conventional CSD type image pickup element. In the drawing, 1 denotes a photoelectric conversion element such as photodiode or the like arrayed in two dimensions, 2 denotes a transfer gate for reading a charge from the photoelectric conversion element, 3 denotes a CSD channel, 4 denotes a CSD transfer electrode, and the transfer gate 2 and the CSD transfer electrode 4 are constituted mostly of the identical electrode. A reference numeral 5 denotes a vertical scanning line, and all the CSD transfer electrodes 4 are connected horizontally through one vertical scanning line 5. A reference numeral 6 denotes a storage part for storing a charge transferred from the photoelectric conversion element 1 temporarily, 7 denotes a control gate of the storage part 6, 8 denotes an output part, 9 denotes a horizontal CCD (charge coupling device) arrayed horizontally, 10 denotes a CSD scanning circuit, 11 denotes an interlacing circuit, 12 denotes a TG scanning circuit. An operation will be described, next. FIG. 7 shows a vertical section of the CSD image pickup element and potentials typically. Now, when the transfer gate 2 during a horizontal blanking period is selected by the TG scanning circuit 12, a signal charge (Q.sub.sig) is read to the CSD channel 3. In the drawing a vertical scanning line 5c is selected, and the charge is read from the photodiode 1 with hatching (FIG. 7(a)). Next, the charge Q.sub.sig is transferred by CSD during a horizontal reading period (FIG. 7(a) to (d)). In this case CSD is driven by the CSD scanning circuit 10 to operate similarly to a four-phase CCD. Then, the charge Q.sub.sig is stored in the storage part 6 and transferred to the horizontal CCD 9 through the control gate 7 during the next horizontal blanking period (FIG. 7 (f)). FIG. 8 represents one example of the operation during one horizontal period.
However, a smear Q.sub.smear due to stray component arises generally in the aforementioned solid image pickup element, which may cause a deterioration of the picture quality. That is, in the CSD type solid image pickup element, the signal charge from the photoelectric conversion element 1 is transferred to the storage part 6 by using one horizontal reading period entirely, therefore charges caused by the smear having arisen near the photoelectric conversion element 1 are all transferred to the storage part 6, thus joining with the signal charge. Consequently, the signal (Q.sub.sig +Q.sub.smear) becomes that in which a smear component is superposed, thus deteriorating the quality as a pickup image.